We obtained cloned full-length DNA copies of influenza viral gene segments that code for the nonstructural proteins (NS), membrane protein (M), neuraminidase (NA), nucleoprotein (NP), hemagglutinin (HA), and polymerase protein (P3). Cloned complete DNA sequences were inserted into the late region of SV40 and the recombinant DNA was propagated in the presence of a tsA SV40 helper. SV40-influenza DNA recombinants that synthesize either the (+) or the (-) full-length strand of influenza viral transcripts were isolated for use in allele replacement, that is, conversion of cloned DNA back into virion RNA. As a first step, we employed the technique of "marker rescue" under antibody selection using the H1N1 subtype of an influenza virus (strain WSN) and an SV40-HA (H3) or SV40-NA (N2) recombinant that produced the (+) strand of influenza viral RNA transcripts in a coinfected cell. Similarly, "marker rescue" of an influenza ts mutant defective in P3 was tested with an SV40-P3 recombinant that transcribed the (+) strand of influenza viral P3 RNA. Preliminary characterization by immunologic techniques and/or gel analysis did not identity progeny that contained a substituted allele corresponding to a rescued viral gene and seven remaining genes from WSN virus. Experiments using recombinants that transcribe the (-) strand viral RNA are under way to determine whether this approach will prove successful for use in allele replacement.